Ransomh.] TJlC Great Valley. 405 



these questions for the particular area under discussion, an at- 

 tempt will be made in the succeeding section to carry the same 

 general plan of investigation farther, and to arrive at more gen- 

 eral results by admitting other regions into the discussion. 



The Hypothesis Not Necessary. — There is probably no more 

 obvious and general law in dynamical geology than the statement 

 that the elevated regions are exposed to denudation, and depressed 

 regions are areas of deposition whenever they are so situated as to 

 receive sediment from an adjacent land mass. By the operation of 

 forces of which we are profoundly ignorant, a portion of the earth's 

 crust is elevated above the general level, while at the same time an 

 adjoining portion may be depressed below it. A cycle of erosion, 

 transportation, and deposition is initiated, whose activities, at first 

 less energetic than the original disturbing forces within the spheroid, 

 gradually equal and then surpass them in intensity, and finally 

 reduce all again to the original mean level that preceded the begin- 

 ning of the cycle. It is certain that isostasy can not account for the 

 initiation of such a cycle. Being purely a theory of readjustment 

 to a former state of equilibrium, it obviously can not be appealed to 

 as causing an initial disturbance of such equilibrium. If we con- 

 sider an original deformation of the lithosphere produced by some 

 other or unknown cause, then it is possible to imagine isostatic 

 adjustment as taking place by a series of secular oscillations of 

 diminishing amplitude ; but, as Woodward has pointed out, such a 

 system would come with geologic rapidity to a state of final rest. 

 Geology, however, gives little support to such a view of final peace 

 and quietness. Mountain systems have risen and have been 

 swept away, while many of the greatest mountain ranges and 

 loftiest table-lands of the present day are, as is well known, of com- 

 paratively modern date ; and it is manifestly impossible to regard 

 their elevation as due to enfeebled remnants of oscillations which 

 were set up once for all during the birth-throes of Archaean or pre- 

 Archaean ancestors. 



Without at present going further into these general questions, it 

 would seem that we are justified in assuming the present activity of 

 very energetic forces within the spheroid, capable of elevating and 

 depressing both large and small areas of the earth's surface, and 



